On a lead, let off to frighten (7)
I believe the answer is:
startle
'frighten' is the definition.
(thesaurus)
'on a lead let off' is the wordplay.
'on' means one lot of letters goes inside another (as in clothing \'on\' a person).
'a lead' becomes 'star' ('star' can be a synonym of 'lead'**).
'off' is an anagram indicator.
'let' anagrammed gives 'tle'.
'star' going within 'tle' is 'STARTLE'.
'to' acts as a link.
(Other definitions for startle that I've seen before include "Alarm (someone)" , "Cause to feel sudden alarm" , "Surprise with starlet" , "Rattles (anag.)" , "Take by surprise" .)